In the manufacture of fiber from molten material, it has been common practice to use electrically heated bushings made of precious metals including platinum, rhodium, palladium, ruthenium, iridium and alloys thereof. The bushings are electrically heated by their own resistance and are usually box-like, open on the top and comprise an orifice plate containing hundreds or thousands of orifices, with or without nozzles or tips welded or formed thereon, as shown by U.S. Pat. Nos. 4,207,086 and 4,078,413, which disclosures are hereby incorporated by reference.
As the molten material emerges from the orifices or tips, a meniscus or cone of molten material is formed below each orifice or tip from which a fiber is pulled continuously. This is the objective, but if the fiberizing quality of the glass reaching the bushing, and particularly the orifice plate or tip plate, i.e. the temperature of each meniscus is not within the fiberization range, or the molten glass contains small stones, other inclusions or chords, one or more fibers will break, requiring a costly stoppage of desired fiberization from that bushing and a beading down and restart to achieve the desired fiberization. By desired fiberization is meant that the bushing is operating making the desired fiber for the product being produced. To remove the heat from the meniscus and fiber that must be removed to cool the molten or plastic fiber so that it will have integrity and strength to endure the remainder of the process of making the fiber product, cooling members are located close to the orifices or nozzle tips. These cooling members can be either cooling tubes like shown in U.S. Pat. Nos. 4,397,665, 5,244,483 and 6,196,029, the disclosures of which are hereby incorporated by reference, or cooling fins as are well known in the fiber industry.
Occasionally, and sometimes frequently, a fiber will break beneath the bushing for various reasons that are known. When a fiber break occurs, the loose fiber soon causes other fibers to break and soon all, or most, fibers being formed beneath the bushing are broken, a stoppage of desired fiberization. This is called a “breakout” in the industry. After a breakout begins, it is necessary to wait a short time, usually tens of seconds up to a few minutes, for beads of molten glass to form beneath each bushing orifice or tip, and become large enough that they break loose and fall from the bottom of the orifice plate or tip pulling a very coarse fiber, called a primary fiber, onto the floor, into a scrap bin, basement or scrap bin beneath the forming room floor. This is normally called “beading out” in the industry. Once beaded out, or as soon as the operator is available, the operator or starting equipment can then restart a strand containing the primary fibers into a chopper or winder and again begin making the desired product. Detectors for detecting when a breakout is occurring are known as evidenced by U.S. Pat. Nos. 4,130,406, 4,229,198, 4,342,579, 3,432,580, 4,401,452, and 4,925,471.
When the bushing is running good product the fibers are moving away from the bottom of the bushing at a speed of thousands of feet per minute. This downward movement at this speed, of an array of hundreds or thousands of fibers, creates, due to friction between the air surrounding the fibers and the surface of the fibers, a partial vacuum (lower pressure zone) by pulling a stream of air downward. The partial vacuum causes a flow of cooling air from the surroundings into the array close to the orifice plate and tips of the bushing. This flow of inspirated air coming from outside the array of fibers cools the tips, meniscuses and the newly formed fibers. The cooling of the bushing, tips and orifice plate, causes additional electrical power to be applied to the bushing automatically to maintain the set-point temperature.
When the bushing breaks out, this inspirated cooling flow of cooling air stops. At that time several more undesirable things begin. The set point thermocouple begins to heat up because of the loss of cooling air and as it does, the controller decreases the electrical power heating the bushing. As the electrical power is decreased during the beading out and hanging periods, the molten glass through-put decreases by 5-15 percent, slowing the flow of molten glass through the well, orifice, between the forehearth leg above the bushing and the bushing causing the temperature of the molten glass in the well, and thus the molten glass entering the bushing, to drop substantially, about 25-75 degrees F. This colder glass coming into the bushing causes the molten glass exiting the orifices to be colder and thus to have a higher viscosity. The higher viscosity glass has more resistance to attenuation when desired fiberization is begun, causing higher stress in the fiber at its weakest point, and it frequently breaks. This is why the break rate is normally highest during the first ten minutes or longer after restart of desired fiberization, particularly as the area of the orifice/tip plate of the bushings has increased to accommodate mote orifices/tips. The larger the area of the orifice plate or tip plate, the greater the tendency to have a larger temperature variance across the orifice plate or tip plate or the tips. It normally takes about ten minutes or longer for the molten glass in, and exiting, the bushing to again reach the desired fiberizing temperature.
The above conditions apply to any molten material and are most costly in the manufacture of so-called “continuous” glass fiber products from molten glass. This condition has been addressed in the past by the use one or more air tubes below an electrically heated bushing to run continuously or intermittently to cause a cooling air stream to be inspirated into the bushing tips, the array of newly formed fibers, beads, or primary fibers below the tips even after the bushing has broken out. This is disclosed in U.S. Pat. No. 4,662,922. This improved the break rate, but is not sufficient, particularly on large bushings having one or two thousand or more tips, to prevent the problems described above.
It is also known in electrically heated flat plate bushings having no tips or nozzles such as disclosed in U.S. Pat. Nos. 3,905,790 and 4,229,198, but only orifices or slightly raised lands around one or a few orifices, to blow cooling air upward onto the orifice plate during fiberizing, beading out and hanging modes, i.e. producing primary fibers, not a desired fiber product. However, flat plate bushings have proven to be costly to operate and are to date are only practical for making coarse fibers having a diameter of 19 or more microns. Attempts to operate a tip type bushings having cooling members mounted beneath the orifice plate using the upward air flow like that used on flat plate bushings have proven to be deleterious to good fiberizing efficiency. But it is known to use upward air flow cooling to try to replace conventional cooling members like cooling fins and or cooling tubes normally mounted beneath the orifice plate of a bushing as disclosed in U.S. Pat. Nos. 4,321,074 and 4,362,541, but these must not have proved successful because the glass fiber industry still uses cooling tubes and cooling fins beneath the bushing.
In the manufacture of continuous glass fibers, melting furnaces are typically used to melt batch, refine the molten glass, and to feed molten glass through one or more forehearths and usually a plurality of bushing legs to the bushings. It is extremely important, to achieve a very low break rate, bushing breakout rate, that the molten glass coming to the bushings is fully melted and uniform in temperature and chemistry. Mixing in the molten glass is mainly dependent upon maintaining desired temperature gradients in the melting furnace. There are typically hundreds of thousands of pounds of molten glass, often about 500,000 pounds, in a typical melting furnace system for making continuous glass fibers. With this much molten material, the melting furnace and delivery system has great momentum and inertia, i.e. it is difficult and takes considerable time correct a change in the molten glass reaching the bushings following a furnace upset. A furnace upset is anything that makes a significant change in the way the melting furnace is operating, including a significant change in the throughput of molten glass through the delivery system, including the bushings. In the past it has been noticed that when a plurality of bushings were stopped from making desired fiber product and put into a hanging mode, to permit a chopper that had been pulling strands of fibers from the bushings to be rebuilt, that after a few minutes the conditions inside the melting furnace would change and that the automatic burner controls for the melter were changing conditions of the burners responding to the change(s) in the furnace. This is necessary, but not desirable. Although improvements in melting furnace control and stability have been made through the decades that large melting furnaces have existed, frequent furnace upsets or disturbances still exist result in lower productivity and higher manufacturing costs.